1. Field of the Invention
The present invention relates generally to the field of electrical protective devices and, more particularly, to an electrical protector having balanced heat coils to protect telecommunications circuits against excessive currents.
2. Description of the Related Art
In telecommunications systems, such as the public switched telephone network (xe2x80x9cPSTNxe2x80x9d), it is common to provide electrical protectors at central offices for each incoming line. These protectors typically provide protection against excessive voltages caused, for instance, by lightning, equipment failure or downed power lines, as well as protection against sneak currents. Sneak currents are produced at a relatively low magnitude compared to the excessive voltages mentioned above and are generally not strong enough to damage the telecommunications equipment if allowed to flow briefly. If allowed to persist, however, sneak currents may indeed pose a serious safety hazard by generating enough heat to char conductor insulation and start a fire, or otherwise cause damage. Sneak currents may be caused, for instance, by accidental interference between adjacent telephone and power lines, accidental shorting of wires in the central office, or a defect causing the protected telecommunications equipment to draw excessive sneak current.
To protect against these undesirable sneak currents, such protectors are typically provided with a pair of heat coils, one to protect the ring conductor and the other to protect the tip conductor associated with each telephone station apparatus. Each heat coil includes an insulated high resistance wire that is wound about a metal sleeve that is held in a predetermined position on a conductive line pin by a fusible bonding material (e.g., solder) having a predetermined melting point. One end of the insulated conductive wire is typically welded to the sleeve and the other end to the line pin. A spring typically biases the sleeve toward an electrical contact with a source of ground, the biasing force of the spring being less than the holding force of the fusible bonding material or solder. When excessive currents occur on the line and persist, sufficient heat is generated by the insulated conductive wire to melt the solder and release the sleeve, which sleeve is urged by the spring into electrical contact with the source of ground potential to ground the line.
While the above-mentioned protector design provides satisfactory protection against damage associated with excessive sneak currents, the ring and tip conductor circuits are not necessarily grounded at the same time. That is, assuming, for example, that an excessive sneak current causes the heat coil sleeve in the tip conductor circuit to electrically contact the ground potential, the sneak current may still flow from the ring conductor through the protected equipment and back through the tip conductor to ground if the differential voltage between the tip and ring conductors is not zero. The heat coil associated with the ring conductor will not ground the ring conductor circuit until the temperature associated with this excessive sneak current causes the solder holding the sleeve to melt and ground the ring conductor circuit. Therefore, it would be advantageous to provide a balanced heat coil protector where the ring and tip conductor circuits are grounded at the same time.
Additionally, conventional heat coil electrical protectors often provide undesirable interference of data being transmitted to and from the protected equipment (e.g., from the central office to the customer, or vice versa). It would also be desirable to provide a heat coil protector that eliminated such interference.
The foregoing and other objects and advantages are achieved in accordance with the present invention through the provision of a balanced heat coil electrical protector, which simultaneously grounds and shorts the ring and tip conductor circuits when an excessive sneak current occurs in the ring conductor circuit, the tip conductor circuit, or both.
In a preferred embodiment, a balanced heat coil protector is provided for protecting telecommunications equipment from excessive sneak currents through a telecommunications circuit having a ring conductor and a tip conductor. The balanced heat coil protector comprises a dielectric base and a dielectric housing adapted to be fixed to the base. An electrically conductive grounding pin is supported within and extends through the base for providing a path to ground potential. A first electrically conductive line pin and a first electrically conductive central office pin are supported in the base and extend therethrough, the first line pin and first central office pin being electrically connected to each other by a first conductive wire to provide a normal circuit current path from the first line pin to the first central office pin. A second electrically conductive line pin and a second electrically conductive central office pin are also supported in the base and extend therethrough, the second line pin and second central office pin being electrically connected to each other by a second conductive wire to provide a normal circuit current path from the second line pin to the second central office pin. A sleeve having electrically conductive flanged end portions is supported in an initial position within the housing on the grounding pin by a fusible material and is disposed above at least one of the first line and central office pins and at least one of the second line and central office pins, and the first and second conductive wires are wound about an outer surface of the sleeve. The fusible material melts in response to an excessive sneak current in either the ring or tip conductors causing the sleeve to move from the initial position to a grounding position in contact with at least one of the first line and central office pins and at least one of the second line and central office pins to simultaneously ground the ring and tip conductors.
In another preferred embodiment, a balanced heat coil protector is provided for protecting telecommunications equipment from excessive sneak currents through a telecommunications circuit having a ring conductor and a tip conductor. The balanced heat coil protector comprises a dielectric base and a dielectric housing adapted to be fixed to the base. An electrically conductive grounding pin is supported within and extends through the base for providing a path to ground potential. An electrically conductive grounding plate is positioned within the housing above the base and is electrically connected to an end of the grounding pin. First, second, third and fourth electrically conductive pins are supported in the base and extend therethrough. First and second heat coil contacts are disposed within the housing above the base, the first heat coil contact being electrically connected to the first pin and the second heat coil contact being electrically connected to the third pin. The first and second heat coil contacts and grounding plate are positioned within the housing to avoid contact with each other. A first electrically conductive sleeve having flanged end portions is supported in an initial position within the housing on the first pin by a fusible material and disposed above the grounding plate and the second heat coil contact. A first conductive wire is wound about an outer surface of the first sleeve with one end being electrically connected to an end portion of the sleeve and the opposing end of the first wire being electrically connected to the second pin to establish a normal current path from the first pin through the sleeve and wire to the second pin. A second electrically conductive sleeve having flanged end portions is supported in an initial position within the housing on the third pin by a fusible material and disposed above the grounding plate and the first heat coil contact. A second conductive wire is wound about an outer surface of the second sleeve with one end being electrically connected to an end portion of the second sleeve and the opposing end of the second wire being electrically connected to the fourth pin to establish a normal current path from the third pin through the second sleeve and second wire to the fourth pin. The fusible material melts in response to an excessive sneak current in the ring conductor causing the first sleeve to move from the initial position to a grounding position in contact with the grounding plate and second heat coil contact to simultaneously ground the ring and tip conductors, and the fusible material melts in response to an excessive sneak current in the tip conductor causing the second sleeve to move from the initial position to a grounding position in contact with the grounding plate and first heat coil contact to simultaneously ground the ring and tip conductors.
The foregoing specific objects and advantages of the invention are illustrative of those that can be achieved by the present invention and are not intended to be exhaustive or limiting of the possible advantages which can be realized. Thus, these and other objects and advantages of this invention will be apparent from the description herein or can be learned from practicing this invention, both as embodied herein or as modified in view of any variations which may be apparent to those skilled in the art. Accordingly, the present invention resides in the novel parts, constructions, arrangements, combinations and improvements herein shown and described.